Introduction 

If you’ve ever compared results from OpenRoads Designer (ORD) with those from a spreadsheet or another software, you may have noticed differences—especially in hydraulic grade line (HGL) and flow depth calculations. This can be confusing, particularly if your organization has long relied on a trusted spreadsheet or tool for stormwater design. 

In ORD, the Drainage and Utilities calculations are powered by a built-in OpenFlows StormCAD engine that allows hydraulic analysis of drainage networks with up to 100 inlets. For larger networks or to activate more advanced hydraulics functionality please read this article. 

The GVF‑Rational (StormCAD) solver embedded in ORD is intended for peak‑flow storm drainage design. It applies Rational Method hydrology to compute design flows and uses gradually varied flow (GVF) hydraulics to evaluate changes in water depth in pipe networks. This article refers specifically to this default solver in ORD. 

Many spreadsheets and older software tools use open channel flow calculations based on Manning’s equation and uniform flow, which means they assume the flow is steady and consistent (at normal depth) throughout the pipe or channel. This approach is straightforward and familiar but doesn’t always reflect the real-world behavior of stormwater systems, especially when conditions change or pipes surcharge.  

In fact, if you’re using a macro-enabled spreadsheet or a legacy DOT tool, it’s likely that GVF calculations are not being performed at all. Sometimes the calculations in these spreadsheets are not documented, making it difficult to know exactly what assumptions are being made.

This article is designed to help you understand why ORD drainage results may differ from those of other tools, why they can be considered an improvement, and how to adjust calculation options if you must match the approach you are used to. With this, you can gain confidence in your hydraulic results. 

Why Results May Differ: Uniform Flow vs. GVF 

Uniform Flow Calculations: 

• Assume the water surface is parallel to the bottom of the pipe.
• Require a prismatic conduit and steady, non-accelerating flow.
• Use normal depth for all calculations.
• Are common in spreadsheets and older tools. 

Gradually Varied Flow (GVF) Calculations: 

• Account for changes in depth, velocity, and flow profile type along the pipe or channel.
• Reflect real-world effects like backwater, surcharging, hydraulic jumps and transitions.
• Are used by ORD for more accurate modeling. 

If your spreadsheet or software uses uniform flow, it may not capture these real-world effects, which is why results can differ from those from ORD. 
 

Comparing Hydraulic Calculators: FHWA Hydraulic Toolbox, HY-8, and ORD  

If you are comparing results from hydraulic calculators such as FHWA Hydraulic Toolbox or HY-8 to those from ORD, it’s important to understand a key difference in how these tools work: 

• FHWA Hydraulic Toolbox analyzes one element at a time—such as a single pipe or culvert—using uniform flow calculations. It does not consider how flow moves through an entire drainage network.
• HY-8 is designed specifically for culvert analysis. It only evaluates the culvert itself, not the upstream or downstream system.
• ORD, on the other hand, analyzes the entire drainage system. This includes everything from catchments and inlets to pipes and outfalls. This allows you to see the continuity of flow from one element to the next, providing a more complete and realistic picture of how your stormwater system will perform. 

How to Change Calculation Options in ORD 

ORD gives you the flexibility to choose between two calculation methods for flow profiles: 

Backwater Analysis:

• Uses GVF calculations for a detailed, realistic profile.
• Accounts for non-uniform flow, boundary conditions, and transitions. 

Capacity Analysis: 

• Uses a simplified approach based on uniform flow principles for partly full pipes
• May not exactly replicate spreadsheet results, as tailwater conditions or backwater may cause flow depth to be greater than a conduit’s normal depth 

Define Calculation Options in ORD: 

1. Go to Drainage and Utilities > Analysis > Calculation > Options. 
2. Make sure Active Numerical Solver is set to: GVF-Rational (StormCAD) 
3. In the properties of the active solver, find the “Flow Profile Method” setting. 
4. Select either “Backwater Analysis” (for GVF) or “Capacity Analysis.” 
5. Run the calculation and compare the results.

This enables you to see how the results differ and to choose the method that best fits your needs or matches your organization’s expectations. 

Below is a simplified explanation of the differences between Backwater Analysis and Capacity Analysis. For a more detailed explanation, please read this article.  

How Backwater Analysis Works (GVF Calculations) 

Backwater Analysis is ORD’s advanced method for modeling how water behaves in pipes and channels. Instead of assuming the water surface is always parallel to the pipe bottom, it calculates how the water depth and hydraulic grade change in the system. 

How does it work? 

• ORD starts at a known point (usually the downstream end). 
• It uses the principles of gradually varied flow (GVF), which means it considers how depth, velocity, and energy change along the pipe. 
• The solver steps along the pipe, calculating the water surface at each point, taking into account things like slope, pipe size, roughness, and downstream conditions. 
• This method can show effects like backwater (when water backs up due to downstream restrictions), surcharging (when pipes run full), hydraulic jumps and transitions between different flow regimes.
• For steep pipes with supercritical flow, the solver assumes critical depth at the upstream end of a conduit and determines the flow profile using frontwater analysis. See here. 

Why use Backwater Analysis? 

• It gives a much more realistic picture of what’s happening in your stormwater system. 
• It helps you identify potential problems, like flooding or surcharging, that uniform flow calculations might miss. 

How Capacity Analysis Works  

Capacity Analysis in ORD is a simplified approach that uses uniform flow principles, but it does not simply apply normal depth everywhere or match spreadsheet results exactly. Instead, it uses a backwater approach with profile types, depending on the downstream hydraulic grade. 

How does it work? 

ORD evaluates the downstream hydraulic grade and applies one of three cases: 

Case 1: If the downstream hydraulic grade is less than or equal to the normal depth, StormCAD uses normal depth for the entire pipe.